This disclosure relates generally to methods and systems for steganographically embedding information, and more particularly to a system and method for utilizing a multiple color overlay in a substrate fluorescence mask to embed information in documents and/or images.
Current counterfeit prevention systems are mainly based on the use of digital watermarks, a technique which permits the insertion of information (e.g., copyright notices, security codes, identification data, etc.) to digital image signals and documents. Such data can be in a group of bits describing information pertaining to the signal or to the author of the signal (e.g., name, place, etc.). Most common watermarking methods for images work in spatial or frequency domains, with various spatial and frequency domain techniques used for adding watermarks to and removing them from signals.
For spatial digital watermarking the simplest method involves flipping the lowest-order bit of chosen pixels in a gray scale or color image. This works well only if the image will not be subject to any human or noisy modification. A more robust watermark can be embedded in an image in the same way that a watermark is added to paper. Such techniques may superimpose a watermark symbol over an area of the picture and then add some fixed intensity value for the watermark to the varied pixel values of the image. The resulting watermark may be visible or invisible depending upon the value (large or small, respectively) of the watermark intensity.
Spatial watermarking can also be applied using color separation. In this approach, the watermark appears in only one of the color bands. This type of watermark is visibly subtle and difficult to detect under normal viewing conditions. However, when the colors of the image are separated for printing or xerography, the watermark appears immediately. This renders the document useless to the printer unless the watermark can be removed from the color band. This approach is used commercially for journalists to inspect digital pictures from a stock photo agency before buying un-watermarked versions.
Alternatively, another approach uses ultra-violet (UV) ink rendering to encode a watermark that is not visible under normal illumination, but revealed under UV illumination. The traditional approach, often used in currency notes, is to render a watermark with special ultra-violet (UV) fluorescent inks and to subsequently identify the presence or absence of the watermark in a proffered document using a standard UV lamp. However, these inks are costly to employ, and thus are typically only economically viable in offset printing scenarios, and thus only truly avail themselves of long print runs. Additionally, these materials are often difficult to incorporate into standard electro-photographic or other non-impact printing systems like solid ink printers, either due to cost, availability or physical/chemical properties. This in turn discourages their use in variable data printing arrangements, such as for redeemable coupons, for example.
There is well established understanding in the printing industry regarding the utilization of fluorescent material inks in combination with ultra-violet light sources as employed for security marks, particularly as a technique to deter counterfeiting. However, there remains a long standing need for an approach to such a technique which will provide the same benefit but with lower complexity and cost, particularly in a digital printing environment, using only common consumables.
All U.S. patents and published U.S. patent applications cited herein are fully incorporated by reference. The following patents or publications are noted:
U.S. Patent Application Publication No. 2005/0078851 to Jones et al. (“Multi-channel Digital Watermarking”) describes a system for providing digital watermarks through multiple channels. The channels can include visible, ultraviolet and infrared channels. The non-visible channels can be selected to fluoresce either in the visible or IR/UV spectrums upon the appropriate illumination in the infrared or ultraviolet spectrums. The watermarks in the various multiple channels can cooperate to facilitate watermark detection or to authenticate an object in which the watermarks are embedded.
U.S. Patent Application Publication No. 2003/0005304 to Lawandy et al. (“Marking Articles Using a Covert Digitally Watermarked Image”) describes a marking system for marking an article with an image not visible to the unaided human eye, with the image containing at least one digital watermark. The digitally watermarked image includes both emissive and photoabsorptive protions and is applied using a substance reactive to a predetermined excitation source and exposure to ultraviolet light. Alternate techniques, such as printing with fluorescent inks may be used in combination. The digitally watermarked image is subsequently observable upon exposure to the predetermined excitation source.
U.S. Pat. No. 6,373,965 to Liang (“Apparatus and Mehtods for Authentication Using Partially Fluorescent Graphic Images and OCR Characters”) teaches a system combining a source of ultraviolet light with apparatus for capturing and recognizing either graphic images or characters, or both. In this patent either a visible sub-image, or a fluorescent sub-image, or a combination image, may further contain steganographic (digital watermark) information that is encoded and inserted using conventional techniques. The digital watermark information may be similarly encoded and inserted into visible and/or fluorescent sub-portions and or recombined OCR characters.
U.S. Pat. No. 7,127,112 to Sharma et al. (“Systems for Spectral Multiplexing of Source Images to Provide a Composite Image, for Rendering the Composite Image, and for Spectral Demultiplexing of the Composite Image by Use of an Image Capture Device”) provides methods and systems for spectrally-encoding plural source images and for providing the spectrally-encoded plural source images in a composite image, for rendering the composite image on a substrate, and for recovering at least one of the encoded source images from the rendered composite image. Each source image is spectrally encoded by mapping values representative of each source image pixel to a corresponding pixel value in one or more of a plurality of colorant image planes. The encoding may include the conversion of each source image to a monochromatic, separation image, which is then directly mapped to a corresponding colorant image plane in the composite image. A plurality of source images can thereby be mapped to a corresponding plurality of colorant image planes in the composite image.
The disclosed embodiments provide examples of improved solutions to the problems noted in the above Background discussion and the art cited therein. There is shown in these examples an improved method for creation of a substrate fluorescence mask having background color(s), UV mark color(s), and distraction color(s), to be printed as an image on a substrate containing optical brightening agents. The method includes selecting one or more UV mark colors for the mask such that the UV mark colors exhibit low contrast against the background color(s) under normal illumination and high contrast against the background color(s) under UV illumination. One or more distraction colors are also selected, such that the distraction color(s) exhibit low contrast against the background color(s) under UV illumination and exhibit high contrast against the background color(s) under normal illumination. A distraction pattern, formed from one or more distraction colors, is also selected.
In an alternate embodiment there is disclosed a system for creation of a substrate fluorescence mask to be printed as an image on a substrate for the purpose of embedding information in printed documents. The substrate fluorescence mask includes at least one background color, at least one UV mark color, and at least one distraction color. The system includes a digital printing device for printing the fluorescence mask image on a substrate containing optical brightening agents. The one or more UV mark colors are selected to exhibit low contrast against the background color(s) under normal illumination and high contrast against the background color(s) under UV illumination. The distraction color is chosen to exhibit low contrast against the background color(s) under UV illumination and exhibit high contrast against the background color(s) under normal illumination. One or more distraction patterns are formed from at least one distraction color.
In another embodiment there is disclosed a substrate fluorescence mask to be printed as an image on a substrate containing optical brightening agents in order to embed information in printed documents. The substrate fluorescence mask includes at least one background color, at least one UV mark color, and at least one distraction color. The UV mark color is specified to exhibit low contrast against the background color under normal illumination and high contrast against the background color under UV illumination. The distraction color is specified to exhibit low contrast against the background color under UV illumination and exhibit high contrast against the background color under normal illumination. One or more distraction patterns is also specified, with the distraction pattern formed from at least one distraction color.
In yet another embodiment there is disclosed a computer-readable storage medium having computer readable program code embodied in the medium which, when the program code is executed by a computer, causes the computer to perform method steps for creation of a substrate fluorescence mask having one or more background color(s), one or more UV mark color(s), and one or more distraction color(s), to be printed as an image on a substrate containing optical brightening agents. The method includes selecting background color(s) and also selecting UV mark color(s) for the mask such that the UV mark color(s) exhibits low contrast against the background color(s) under normal illumination and high contrast against the background color(s) under UV illumination. At least one distraction color is also selected, such that the distraction color exhibits low contrast against the background color(s) under UV illumination and exhibits high contrast against the background color(s) under normal illumination. One or more distraction patterns, formed from distraction color(s), is also selected.